1. Field of the Invention
This invention relates to automatic nulling circuitry, and more particularly to a nulling circuit for isolating and detecting transient disturbances in the rate of pressure change sensed by a pressure sensing apparatus.
2. Description of the Prior Art
In many applications it is important to detect the presence of small leaks in a vacuum system (the term "vacuum" as used herein refers to any low pressure system, and is not limited to an absolute vacuum). A common method for detecting slow leaks makes use of the leak to establish a short term transient disturbance in the pressure within the vacuum chamber. If such a disturbance is detected, the presence of a leak is indicated.
Various methods may be used to establish the initial disturbance. According to one method, the leak is first found with a tracer liquid or gas, and then plugged. As the chamber will continue to be pumped after the leak has been plugged, a transient disturbance to the steady state pressure condition is established which can then be detected to indicate the presence and magnitude of the leak. In another technique, the outside of suspected areas on the vacuum chamber is subjected to a small blast of a lighter gas, such as helium, which passes through any leaks in the area and into the vacuum system. The introduction of the lighter gas also causes a pressure disturbance, which when detected indicates the presence of a leak.
One problem in detecting the relatively small pressure disturbances resulting from small leaks is that such pressure changes can be masked by larger pressure changes in the system. For example, rapid pressure changes take place as the system is pumped down from atmospheric pressure to the desired vacuum level. In order to detect small leaks during this period of time, a leak detection system must be operative over the full range of pressures from atmospheric to the desired vacuum level, and must also be able to discriminate between the large rate of pressure change due to pumping of the system, and the relatively small rate of change due to the leak. Once the vacuum chamber has been largely evacuated, it is frequently found that its internal pressure does not stabilize at a steady level but rather continues to vary, creating a need for continual vacuum pumping. Such instability may result from a drift in the pump speed, from long term changes in leak rates, or from the vaporization of volatile materials such as water within the chamber, otherwise known as outgassing. An ideal small leak detector should be able to discriminate between small leaks and the affects of the foregoing phenomena. Furthermore, the detector should be capable of eliminating the effects of gross leaks, which otherwise could mask the presence of small leaks.
Mass spectrometers have sufficient sensitivity over a large pressure range to function as effective small leak detectors, and have been used as such in the prior art. However, mass spectrometers have been found to saturate under gross leak conditions, and require considerable periods of time to again become operable.
An improved small leak detector is disclosed in U.S. Pat. No. 4,106,350, issued Aug. 15, 1978 to Richard T. Morris and Arthur H. Wildvank. According to the teachings of the patent, a very thin resistance wire is lodged in a gas passage way within the evacuated chamber. The resistance wire is sensitive to its ambient pressure, and in effect is a form of thermistor. The resistance of the wire is determined by its temperature, which in turn is dependent upon the temperature and thermal conductivity of immediately surrounding gas and the ability of the gas to conduct heat away from the wire. Since transient changes in the gas temperature may accompany pressure changes, in accordance with the equation of state, pressure readings are obtained by using the wire to sense the thermal properties of the surrounding gas.
The patent discloses electrical circuitry which is capable of responding to pressure changes sensed by the wire, and providing desired readouts. This circuitry is illustrated schematically in FIG. 1. The pressure sensitive resistance wire is indicated by block 2. A voltage source 4, preferably of the bridge rectified, tracking regulator type which is relatively insensitive to variations to the line voltage, produces an output voltage sufficient to drive a desired level of current through the resistance wire 2. That wire is connected as one leg of a Wheatstone bridge circuit 6, the normally balanced output of which is supplied to a transient readout section enclosed in dashed lines and indicated by numeral 8, and also to a pressure readout section also enclosed in dashed lines and indicated by numeral 10.
Transient readout section 8 comprises a differential preamplifier 12 having positive and inverted inputs which receive the normally balanced output of bridge network 6, an amplifier 14 which receives the output of preamplifier 12 and includes a feedback connected null circuit 16, a second amplifier 18 which amplifies a signal from null circuit 16, and a transient readout means 22 such as an analog current meter, which records the output level of amplifier 18 or sounds an alarm in response thereto, as required.
The steady state portion of the circuit comprises a differential preamplifier 24 which is connected to amplify an unbalanced signal produced by bridge circuit 6, an amplifier 26 connected to the output of preamplifier 24, and a steady state pressure readout device 28, such as a digital voltmeter, which is actuated by amplifier 26.
Null circuit 16 in transient readout section 8 is connected to balance the output of amplifier 14 with its input. Amplifier 14, and null circuit 16, however, have a predetermined response time that introduces a delay in the propagation of an input signal. As a result, steady state pressure conditions are nulled out, while transient disturbances in the pressure within the chamber create an imbalance between the input and output of amplifier 14. This imbalance is transmitted by null circuit 16 for amplification by amplifier 18, the output of which is supplied to transient readout device 22.
The reference patent accordingly provides a means for isolating the low level pressure effects of small leak detection by the use of automatic null circuit 16. However, the circuit has been found to be less then fully effective for small leak detection over large pressure ranges because of a tendancy of preamplifier 66 to saturate under extended changes of pressure. Accordingly, there is still a need for a device capable of isolating and detecting the relatively small transient disturbances in the rate of pressure change which accompany testing for small leaks.